How low must a quota be in effect to have an impact? Using a demand-and-supply diagram, illustrate

Question:

How low must a quota be in effect to have an impact? Using a demand-and-supply diagram, illustrate and explain the net welfare loss from imposing such a quota. Under what circumstances would the net welfare loss from an import quota exceed the net welfare loss from an equivalent tariff (one that results in the same price and import level as the quota)?